Public address systems have been configured traditionally with multiple speakers that are connected together and driven with a common signal, or combined together as multiple networks or zones with a common signal per zone. The common signal originates from one or more sources of audio signal selected for transmission to all speakers, or to all speakers in a zone.
Typically, a public address system is configured as a system in which the amplifiers are colocated with the speakers, that is, the amplifiers are located in the same enclosure as the speakers. A user can adjust the volume of the speakers at the amplifier. The design is simple. A signal from the same source is transmitted to each amplifier. If the amplifiers are distributed throughout the building, different listeners can adjust the volume of the speakers to suit the environment they are in. For example, a listener in a noisy machine shop can adjust the volume to a higher level than a listener receiving the same signal in an office.
U.S. Pat. No. 4,922,536 discloses frequency division (FDM) and/or time division multiplexing (TDM) to digitally transmit audio signals from multiple microphones to a control booth, and to digitally transmit audio signals from the control booth to speakers. At each end of the digital transmission, the digital signals are converted to analog signals for processing. The control booth provides the control for all of the speakers. In another example, use of a microprocessor in a computing system to control routing of audio signals on a computer bus is shown in U.S. Pat. No. 4,862,159. In both of these audio systems, the speakers are dumb devices, that is, there is no digital audio processing at the speakers themselves.
Another example of a distributed speaker system is disclosed in U.K. Patent Application GB 2,123,193A which discloses a speaker system having a master station and remote speakers. Each of the remote speakers has a unique address, and the volume of each speaker can be individually adjusted. However, each speaker requires a respective amplifier that is integrated with the speaker. The amplifier also acts as a switching device to turn the speakers on and off
Thus, it is desirable to provide more flexibility in a speaker system network by using separate audio signals at each speaker in the network. For example, an operator at a central point may wish to transmit a message to only selected speakers in a network, or in multiple networks or zones, rather than to all speakers in a network or zone. Further, it is desirable to maintain amplifiers for each of the speakers in a speaker system network in a central location. Thus, the remote units are less expensive and simpler to maintain.
It is also desirable to provide separate volume control for each speaker, and to selectively broadcast the audio signal to selected speakers in the network system. For example, it is desirable for a public address system to remotely adjust the volume at selected speakers and selectively broadcast to the speakers.